Ultrafiltration Water Treatment Plant

Challenges

The original plant was constructed in 1984 with a major expansion in 1990. It is operated by Creston City Water Works (CCWW) and provides water to Creston and Southern Iowa Rural Water Association (SIRWA), which serves all unincorporated areas in 7 counties and portions of 7 other counties in southwest Iowa.

The plant treats water from Twelve Mile and Three Mile Reservoirs; the water is particularly difficult to treat, especially at certain times of the year when quality can change rapidly. CCWW and SIRWA needed to expand the plant to serve a growing customer base, as well as improve water quality and plant operations. The construction phase of this project was particularly challenging because it was necessary to keep the plant in operation while improvements were underway. To accomplish this, the new systems were implemented in phases.

Membrane Fibers

Solution

The $12 million construction project included improvements to every part of the treatment plant, including pumping, chemical feed systems, clarification, and filtration. One major component involved conversion of the conventional rapid sand filters to an ultrafiltration system using submerged, hollow fiber membranes – a highly efficient filtration system that consistently produces clean water with a very low turbidity. Bundles of hollow fiber membranes are suspended in the feedwater and operate under a slight negative pressure created within the membrane fibers by permeate pumps. This negative pressure draws permeate (filtered water) through the membranes, leaving contaminants behind.

Four of the twelve sand filters were converted to membrane filtration; a fifth was converted to store backwash and cleaning water for the membrane system. Those four membrane filters replace twelve existing gravity sand filters of the same size and increase the treatment capacity by 33%. While the membrane system is only 33% of the footprint of conventional filters, the plant capacity is being increased from 6 million gallons per day (MGD) to 8 MGD, with capability of being expanded to 9 MGD.

The remaining seven filters were converted to granular activated carbon (GAC) contactors which remove taste and odor causing compounds and other organic contamination. The contactors include a 9-ft deep carbon bed, new block-style underdrains with media retaining cap, and new backwash troughs.

Pre-treatment improvements prior to membrane filtration include a new coagulant building to house chemical feed systems and storage tanks, and conversion of four Infilco Degremont Pulsator® clarifiers to SuperPulsator® clarifiers. This included installation of settling baffles in the clarifiers to increase capacity and improve performance.

Membrane system increased treatment capacity from 6 to 8 MGD

Outcome

CWW has been able to assure its customers of its interest in maintaining quality service. CWW and SIRWA have been able to file timely applications for project funding and increase their chances for securing the needed financing. The engineering evaluation and pilot study are providing valuable information that will allow the utility to make sound technical, financial, and managerial decisions.

Variable frequency drive

Recognition

FOX Engineering Associates, Inc. and the city of Creston Water Works received the American Council of Engineering Companies (ACEC) 2nd Place Award for Engineering Excellence in the Water and Wastewater Category for the membrane filtration project.